FIG. 10 shows a traditional liquid treatment device. A first electrode 801 and a second electrode 802 are disposed in liquid 803 (for example, water), and a pulse power supply 804 applies a high-voltage pulse across the electrodes 801 and 802 to vaporize the liquid 803 and generate a plasma 805. Here, contaminants and other foreign substances contained in the liquid 803 are decomposed as the plasma directly contacts the liquid 803. Simultaneously, for example, highly oxidative components, such as hydroxyl radicals (OH radicals) and hydrogen peroxide, are generated and decompose contaminants and other foreign substances contained in the liquid 803 by reacting with these substances. Among the radicals that occur as a result of plasma generation in the liquid 803, OH radicals are known to have particularly high oxidative power, and enable decomposition of persistent organic compounds dissolved in the liquid 803.
A problem of the traditional liquid treatment device, however, is that a high voltage needs to be applied to vaporize the liquid, and the liquid treatment takes a long time because of the poor plasma generation efficiency.
As a technique to improve plasma generation efficiency with low applied voltage, a liquid treatment device is known that incorporates externally introduced gas between electrodes (see JP-A-2000-093967). In the liquid treatment device of this related art (see FIG. 11), a pulse voltage is applied between an anode electrode 901 and a rod-like cathode electrode 902 with a liquid to be treated 903 and a gas 904 (for example, oxygen) being present between these electrodes. The applied pulse voltage generates a plasma in the gas 904, and decomposition takes place at the plasma surface contacting the liquid to be treated 903. The liquid treatment described in JP-A-2000-093967 enables use of a smaller applied voltage than when gas is absent, and efficiently generates a plasma for liquid treatment.
However, the OH radicals that are generated in such a liquid treatment device are very reactive, and the life is short as they quickly turn into relatively stable hydrogen peroxide. Hydrogen peroxide is known to generate OH radicals by reaction with metals such as copper and iron, or the Fenton's reaction as it is commonly called. The formula (1) below represents a Fenton's reaction with copper ions, in which OH radicals generate as monovalent copper ions react with hydrogen peroxide and turn into divalent copper ions. The divalent copper ions are known to react with hydrogen peroxide to simultaneously produce monovalent copper ions, as represented by formula (2). Formulae (3) and (4) represent Fenton's reactions with iron ions, which simultaneously take place, and produce OH radicals from divalent iron ions, and divalent iron ions from trivalent iron ions. That is, metal ions are known to undergo catalytic reaction in the Fenton's reaction.Cu++H2O2→Cu2++.OH+OH−  (1)Cu2++H2O2→Cu++HO2.+H+  (2)Fe2++H2O2→Fe3++.OH+OH−  (3)Fe3++H2O2→Fe2++HO2.+H+  (4)
A liquid treatment device is known that, by taking advantage of the Fenton's reaction, improves treatment performance by regenerating OH radicals from the altered hydrogen peroxide (see JP-A-2013-138990). In the liquid treatment device of this related art (see FIG. 12), at least one of the electrodes 64 and 65 is configured from metals containing copper or iron. Because of this configuration, applying a voltage to the pair of electrodes 64 and 65 from a high voltage generator 70 causes electrolysis, simultaneously with plasma generation in water. This causes the copper or iron to dissolve out of the electrodes, and produces copper ions or iron ions. These copper ions or iron ions undergo the Fenton's reaction in the water being treated, whereby the copper ions or iron ions react with the hydrogen peroxide produced from OH radicals or other chemical species generated by a plasma, and produce OH radicals. The OH radicals can then react with the organic materials or other substances contained in the water being treated, and the treatment performance of the liquid treatment device can improve.
The liquid treatment device described in JP-A-2000-093967 uses a rod-like positive electrode. The reason for using a rod-like electrode as the positive electrode is that the discharge between a rod-like electrode and a plate electrode occurs more easily, and the energy efficiency improves when a positive voltage is applied to the rod-like electrode. The Fenton's reaction can take place also in such a liquid treatment device when the electrodes use copper- or iron-containing metals as in JP-A-2013-138990.
However, because the metal ions dissolve out of the rod-like positive electrode through electrolysis, the electrode severely wears out, and the life of the rod-like electrode is considerably cut short after a long treatment. In the end, the device is no longer able to stably generate plasma. That is, the system cannot operate for extended time periods.